Means for eliminating phase and frequency modulation



Sept. 16, 1941.

C. W. HANSELL MEANS FOR ELIMINATING PHASE AND FREQUENCY MODULATION Filed Aug. 12, 1959 ATTORNEY Patented Sept. 16, 1941 MEANS FOR ELIlVHNATING PHASE AND FRE- QUENCY MODULATION Clarence W. Hansell, Port Jefferson, N. Y., assignor to Radio Corporation of America, a corporation of Delaware Application August 12, 1939, Serial No. 289,728

(Cl. Z50-20) 12 Claims.

This application relates to a new and improved system for and method of reducing or eliminating components of phase or frequency modulation from an amplitude modulated carrier wave, in such a manner that the amplitude modulationcomponents thereof are retained unaffected by the reducing or eliminating process.

My invention, as applied to amplitude modulated waves and currents, is analogous in some respects to amplitude limiting, which has been applied to phase and frequency modulated waves and currents with very benecial results. The effects of limiting in the case of reception of phase and frequency modulated waves have been studied at considerable length, and a considerable amount of material pertaining thereto will be found in publications.

Whereas, in the reception of phase or frequency modulated Waves it is customary to employ limiting amplifiers, or instantaneous volume control, in order to eliminate all amplitude modulations before nal detection, my present invention contemplates eliminating all phase or frequency modulation before final detection.

In describing my invention reference will be made to the attached drawing wherein:

Figure 1 illustrates diagrammatically the essential elements of a receiver arranged in accordance with my invention, while;

Figure 2 illustrates a modification of the arrangement of Figure 1.

Figure 1 shows a block diagram of a radio receiving system made up of elements for carrying out definite functions each of which is well known in the radio art. These elements as arranged comprise the principal elements included in one form of my invention. In Figure 1, I have shown a receiving antenna A for picking up amplitude modulated radio energy and for delivering this energy to a first unit I of a receiver. This unit includes a heterodyne detector by means of which received currents may be beat down to a lower or intermediate frequency of a value which is readily controllable, just as in any superheterodyne receiver. The first receiver unit should preferably include high frequency selective circuits, high frequency amplification, output or intermediate frequency amplification and any other desirable features commonly used in superheterodyne receivers.

The output of the first receiver unit I0, which is at an intermediate frequency of say F1, is applied to two separate heterodyne detectors I4 and I6. One of these detectors, say I4, is supplied by an oscillator I8 with beating energy, of a frequency say F2, which is lower than the frequency F1. The other detector, I6, is supplied by frequency multiplier 20 with beating energy of a frequency which is a harmonic or multiple of F2, say 2F2, which is higher than F1. The unit IS includes an oscillation generator of the constant frequency type, and, if desired, a frequency multiplier to permit the constant frequency oscillator to operate at a frequency lower than F2, while the unit 20 includes any means for producing wave energy which is a harmonic or multiple of the energyfromV I8. The output of one of the pair of detectors, say I4, is then at a frequency of F1-F2 and the output of the other detector, IE5, is 2F2-F 1. v

The outputs of the pairs of detectors are combined in a single side b-and modulator 24 consisting of a balanced modulator and band-pass filter of the type commonly used in carrier current Wire line telephony and in long Wave transoceanic telephone transmitters. The balanced modulator 24 may be of the Carson type, modified and improved to include present day refinements. In this balanced modulator the applied waves cancel or are shunted out leaving the components resulting from the modulation process. Alternatively,

the modulator may be of the types described inV my United States Patents Nos. 1,838,763 or 2,163,680. The output from the balanced modulator: 24 contains frequencies equal to the sum and difference of the input frequencies from the two detectors.

The difference frequencyis equal to Since 3Fz is a constant, it can be seen that when the frequency of F1 is Varied or modulated, then the difference frequency in the output of tle balanced modulators 24 varies twice as much in cycles per second. As will be seen later, the difference frequency has all of the frequency variation present in the output of the balanced modulator. Consequently, the unit 24 includes a band-pass filter and the band-pass filter is so designed and the relative frequencies so chosen that the difference frequency, with its phase and frequency variations, is eliminated or rejected by the band-pass lter which follows the balanced modulator, in the single side-band modulator system. Thus, we eliminate the effects of all variations in the phase or frequency of F1.

The sum frequencyin the output of the balanced modulator is equal to:

S=(2F2F1)-|(F1-F2) :F2 since F2 is constant we allow this component of of phase or frequency modulation or variation. This result is accomplished because the outputs from the two heterodyne detectors I4 and I6, whichy are added together in the single side-band modulator, contain equal and opposite phase and Vfrequency modulations or variations, Which balance out'tto zero in the output of the single sideband modulator.

On the other hand, assuming that all the detec= tors and the balanced modulator are'lineargdee vices, which they normally are, the amplitude. of,

Vfrequency to pass through the band-pass lter and from it has been eliminated all components may, for example, employ a limiter and phase or frequency modulation detector, at the intermediate frequency, for deriving phase or frequency modulation energy and use the energy so obtained to frequency modulate the rst beating oscillator quency` energy from the intermediate frequency output from the single side band modulator Z4 Will be proportional to the amplituderof-the: carrier Wave picked up by the receiving antenna A; Therefore, amplitude modulations of the carrier wave Will'be faithfully reproducedr intheoutput.A of the single side-band modulator, except, for

p changes in Wave form due to frequency selective circuits which' Wouldpbealik'ein character. for-any amplitude modulation receiving equipment..

The arrangement; of Figure limayv be modied.

as shown in: Eigurei. 21'by..I makingv the.A unit: 214'a heterodyne detector, insteadgofl asingle `side-band modulator, if We choose the frequencies lzand 2F2 so that bothlie above,cr both. lie below the frequency F1. Then. the differencebetween the output` frequencies offunitsv IlliandzIGQwill be constant in spite of; variationsainvli an'd'all' the frequency variations.v wiil: appear in the sum of the outputs from unitsl4andl I5, whichv is rejected bythe frequency selective circuits'in the output of the heterodyne detector.

A desirable feature of my novel method` and means 'is that sincev the side-bandi orthei beat frequency, selected is of 1a frequencyl of'y unvarying phase or frequency irrespective of phase or fre'- quencyY modulation ofthe received amplitude modulated Wave, it is not necessary'to provide meansV for retuningthe receiver inthe event the signalv carrier. drifts VWithin relatively wide limits. l In practicing myvinventon it Will benecessary,Y

I for obtaining best results, to choose input andu output frequencies from the two heterodyne detectorsV I4 and I6 so thatY noney of`V their output" frequencies lie within'the pass band ofthe filterA in-the output ofthe single side-band modulator, or heterodyne detector,`24. In other Words, 'the frequency relations must be chosen to give a mini'- mum of interference to the normal amplitude modulation of the carrier inthe output ofq the single side-band modulator. This can be done readily by one skilledY in the art.

One promising eld for application of my invention is in long distance short Wave telegraph communication.r My system,V by eliminatingef; fectsof'relative frequency changes between .they transmitter and the first heterodyne oscillator ofthe receiver, can eliminate the need for passing received beat note Vsignals into a keyer. unit, for keying output of a. constant frequency oscillator, in order to obtain a constant yfrequency for transmissionl over a wire line. In my system thein.- termediate frequency oscillators.. arevtheV only oscillators which must have constant relative frequency in order to hold a constantbeatnoteout.- put fromgthe receiver. There, oscillatorsbeing close together in the same receiver, may easily be made to hold sufficiently constant-relative frequencies. I

Although I have shown apreferredreceiver arrangement, it will be understood'that all components of phase or` frequency modulation may be reduced or. eliminated by other means.. We

.circuits` ahead of the limiter and apply it to an amplitude modulation detector. Thus, we may accomplishV almostthe same result as in the system.of. Figure. l;

What I claim is:

1. TheA method. of demodulating amplitude modulated? wave energy and limiting vthe effect Vof extraneous phase and frequency modulation of said wave energy which includes the stepsof, f beating said wave gies of! al rst and second harmonicallyV rela-ted and combining the energies resultingv from said beating action to obtaina `resultant of V constant phase andfrequency for demodulation.

energy with oscillatory enerfrequencies purposes.

2. The method of dem'odulating amplitude modulated wave energy and'V eliminating the effect of extraneous: phase and' frequency"modula-V tion of saidWave-energy which'includes the steps of, beating said WaveL energy with oscillatory ene ergy` of a i'l'rst-A and second harm'onicallyrelated' Afrequencies and addingvr the. energies resulting from said beating .actionv to obtainY thesum ofA components thereof"k for dem'odulation purposes.v

3. The method Vof .demodulating amplitude modulatedwave energy and eliminating the effect'ofextraneous phase and'frequency modula-v tion of saidwave energy which includes the stepsL ofgbeating said wave energywithfoscillatory enr-.VA ergy of a rst'andsecond" harmoni'cally related frequencies and' combining .the energies. resulting lfrom said beating action subtractively toobtain the difference of.compcnentsthereofQfor demodulation purposes..

4'.A The method of deznodulatingl amplitude.

modulated Wave energy and eliminating the ef.- fect of extraneousphase and frequency modulation of said wave energy which includes the steps.

of, beatingsaid Wave energy with oscillatory energy df. afirst andv second harmonically- .related` frequencies Vand, adding. the difference frequencyenergyv components resulting from. saidbeating actions to obtaina. resultant of constantphase andfrequency for demodulation purposes.

5. Amethod as recited in claim. lwherein-one of. said energies of harmonically relatedi frequencies is `of a frequency equal totwice the frequency ofthe other thereof. Y n 6. A method as recited rin-vcla-iml wherein said energies ofharmonically related frequencies are both .of lower frequency than the amplitudemod-Y uflated wave energy.

7. A method as-recited in claim modulated wave energy- 8. The method of demodulating; amplitude modulated wave energy and reducing the effect ofv noise componentsv thereon. and4 simultaneously 1 wherein said energies of harmonically related frequencies are` both of higher frequency than the-amplitude:

wave energy with oscillatory energies of rst and second harmonically related frequencies, deriving the difference frequencies from the energies resulting from said beating actions, intermodulating said energies of difference frequency to produce resultant energy in which noise components are symmetrically related with respect to a signal carrying component of constant frequency, and selecting said signal carrying component of constant frequency and a side-band for demodulation purposes.

9. The method of demodulating amplitude modulated wave energy, reducing the effect of noise components thereon and simultaneously substantially eliminating the effect of extraneous phase and frequency modulation of said wave energy which includes the steps of, beating said wave energy With oscillatory energy of a first frequency, beating said wave energy with oscillatory energy of a second frequency harmonically related to said rst frequency, combining the energies resulting from said beating action to obtain resultant energies of frequencies equal to the diiferences between said wave energy and said oscillatory energies, intermodulating said energies of difference frequency to obtain modulated energy wherein said noise components are symmetrically related with respect to one of said oscillatory energies, and selecting a component characteristic of said one oscillatory energy and a side-band for demodulation purposes.

10. In a receiving system adapted to receive amplitude modulated wave energy and eliminate any phase and frequency modulation on said wave energy, the combination of a pair of heterodyne detectors each having inputs and outputs, a pair of harmonically related oscillators coupled withl the inputs of said heterodyne detectors, means for impressing modulated wave energy on the inputs of each of said heterodyne detectors, and wave component combining means having an input coupled with the output of said heterodyne detectors and means for selecting a side-band and carrier from the output of said combining means for demodulation purposes.

11. A receiving system as recited in claim 10 wherein said combining means is a balanced modulator.

12. A receiving system as recited in claim 10 wherein said combining means is a heterodyne detector.

CLARENCE W. HANSELL. 

